This invention relates generally to optical systems. More specifically, it relates to a novel class of spectral power monitors in which optical alignment is actively managed by way of hardware or software control. The spectral power monitors of the present invention are well suited for WDM optical networking applications.
The prevalence of optical communication networks employing wavelength division multiplexing (WDM) has created a particular demand for spectral (channel) power monitors that operate over a broadband optical spectrum range with enhanced wavelength resolution, have sufficiently fast response time, are robust in performance, and are simple and cost-effective in construction.
Conventional spectral power monitors in the art typically use a wavelength-dispersing means, such as a diffraction grating, to separate a multi-wavelength optical signal into a spatial array of spectral channels. An array of optical detectors is positioned to detect the spectral channels in a one-to-one correspondence, thereby providing a power spectrum of the multi-wavelength optical signal. Alternatively, a rotating diffraction grating and an optical detector, or a movable optical detector and a stationary diffraction grating, are used to scan the spectral channels sequentially. Notable disadvantages of the prior spectral monitors employing arrayed optical detectors include stringent fabrication tolerances and painstaking alignment, rendering these devices high in cost and cumbersome in size and operation. Moreover, these prior devices lack dynamic mechanisms to overcome degradation in optical alignment owing to environmental effects, such as thermal and mechanical disturbances, over the course of long-term operation. An inherent disadvantage of the prior spectral power monitors employing a rotating grating (or a scanning optical detector) is that the underlying scanning mechanisms typically involve moving parts (e.g., motors) that require high maintenance and are rather limited in lifetime, thus making them unsuitable for communication networks. The slow scanning speed of these systems further impedes their wide application.
In view of the foregoing, there is a need in the art for a new line of spectral power monitors in which the optical alignment is actively controlled in a simple, robust, and cost-effective construction.
The present invention provides a spectral power monitoring apparatus employing active alignment compensation. The spectral power monitoring apparatus of the present invention comprises an input port, providing a multi-wavelength optical signal and a reference signal; a wavelength-disperser that spatially separates the multi-wavelength optical signal along with the reference signal by wavelength into multiple spectral channels and a reference spectral component having a predetermined relative arrangement (e.g., in a spatial array termed xe2x80x9cspectral arrayxe2x80x9d herein); an array of optical power sensors (termed xe2x80x9coptical-sensing arrayxe2x80x9d herein), including a reference-position-sensing element for receiving the reference spectral component and a plurality of channel-sensing elements for receiving the spectral channels; and an alignment compensation unit that monitors the relative alignment between the spectral array and the underlying optical-sensing array and compensates for any change in the alignment accordingly.
In the present invention, a xe2x80x9cspectral channelxe2x80x9d is characterized by a distinct center wavelength and associated bandwidth, and may carry a unique information signal as in WDM optical networking applications. A xe2x80x9creference signalxe2x80x9d (and the corresponding xe2x80x9creference spectral componentxe2x80x9d) generally refers to any optical signal characterized by a well-defined (and stable) wavelength that does not coincide with any of the wavelengths of the spectral channels under consideration.
In the spectral power monitoring apparatus of the present invention, the optical-sensing array (i.e., a photodiode array) may be configured such that the power levels of the spectral channels impinging on the photodiode array can be related to the electrical signals thus generated by a predetermined conversion matrix, which may be obtained from a calibration. Moreover, selected two (or more) adjacent channel-sensing elements in the optical-sensing array may be utilized such to provide for the reference-position-sensing element.
In one embodiment of the present invention, the alignment compensation unit is servo-based, and in one form may include an alignment-adjusting element for adjusting the alignment of the spectral channels along with the reference spectral component and a processing element. The alignment-adjusting element may be an actuation device coupled to the optical-sensing array for causing it to move, thereby adjusting the relative alignment between the spectral array and the underlying optical-sensing array. The processing element serves to monitor the real-time impinging position of the reference spectral component onto the reference-position-sensing element and to provide control of the alignment-adjusting element accordingly. The alignment compensation unit maintains the reference spectral component at a predetermined location on the reference-position-sensing element by way of servo-control, thereby ensuring the requisite alignment between the spectral array and the underlying optical-sensing array. Such a servo-based alignment compensation unit enables the spectral power monitoring apparatus of the present invention to actively correct for any shift in the alignment that may come about over the course of operation (e.g., owing to environmental effects such as thermal and mechanical disturbances), thereby enhancing the robustness of the apparatus. An additional benefit of using such a servo-based alignment compensation unit is manifested in relaxed fabrication tolerances and precision during initial assembly, rendering the spectral power monitoring apparatus of the present invention simpler and more cost-effective in construction.
In an alternative embodiment of the present invention, the alignment compensation unit is software-based, and may be in the form of a signal processor in communication with the optical-sensing array. The alignment compensation unit includes a predetermined calibration table containing a plurality of conversion matrices, each relating the electrical signals output from the optical-sensing array to the power levels of the impinging spectral channels at a particular impinging position of the reference spectral component. The alignment compensation unit monitors the real-time impinging position of the reference spectral component onto the reference-position-sensing element. At each impinging position of the reference spectral component thus detected, the alignment compensation unit processes the electrical signals produced by the spectral channels impinging onto the optical-sensing array and looks up a corresponding conversion matrix from the calibration table, thereby providing the power levels of the spectral channels. The spectral power monitoring apparatus thus constructed effectively compensates for any shift in the alignment that may arise over the course of operation by way of software control, without involving any xe2x80x9cmovingxe2x80x9d actuation means. This renders the spectral power monitoring apparatus of the present invention a simpler construction with more robust performance.
In the present invention, the wavelength-disperser may be provided by a diffraction grating, such as a ruled diffraction grating, a holographic diffraction grating, a curved diffraction grating, an echelle grating, a transmission grating, a dispersing prism, or other types of wavelength-separating means known in the art. The input port may be provided by a fiber collimator coupled to an input optical fiber. In the event that the multi-wavelength optical signal is carried by the input optical fiber and the reference signal is provided by a reference light source, an optical combiner (e.g., a fiber-optic coupler) may be used to couple the reference light source to the input optical fiber. This provides a simple way of coupling both the multi-wavelength optical signal and the reference signal into the input port. Alternatively, a particular wavelength channel (e.g., a service channel) can be designated to serve as the reference signal on a network level, as might be in WDM optical networking applications, and transmitted along with various WDM signals through the communication system. The spectral power monitoring apparatus of the present invention may further include a beam-focuser, e.g., one or more focusing lenses, for focusing the spectral channels along with the reference spectral component into corresponding spectral spots.
The spectral power monitoring apparatus of the present invention may further employ a polarization diversity scheme, for mitigating any undesirable polarization-dependent effects that may be imposed by one or more polarization-sensitive elements in the system. This may be accomplished by disposing a polarization-separating element (e.g., a polarizing beam splitter) and a polarization-rotating element (e.g., a half-wave plate), along the optical path between the input port and the wavelength-disperser. The polarization-separating element serves to decompose the input multi-wavelength optical signal (along with the reference signal) into first and second polarization components, and the polarization-rotating element in turn rotates the polarization of the second polarization component by 90-degrees. For instance, in the event that the wavelength-disperser is provided by a diffraction grating that provides higher diffraction efficiency for p (or TM)-polarization (perpendicular to the groove lines on the grating) than for s (or TE)-polarization (orthogonal to p-polarization), the aforementioned first and second polarization components correspond to the p-polarization and s-polarization components of the multi-wavelength optical signal (along with the reference signal), respectively. The wavelength-disperser separates the first and second polarization components respectively by wavelength into first and second sets of optical beams, which subsequently impinge onto the optical-sensing array. The first and second optical beams (originating from the two polarization components) associated with each spectral channel may impinge at substantially the same location onto the optical-sensing array. Such a polarization diversity scheme has the advantage of maximizing the diffraction efficiency and therefore minimizing the insertion loss of the system.
As such, the present invention provides a new line of spectral power monitors with active alignment compensation and maximized energy efficiency that are well suited for optical networking applications.
The novel features of this invention, as well as the invention itself, will be best understood from the following drawings and detailed description.